The present invention relates to electrostatic precipitators, and in particular, to controllable power supplies for electrostatic precipitators.
It is known (U.S. Pat. No. 4,290,003) to provide a power supply which has a pair of anti-parallel SCRs (thyristors) coupled through a high voltage transformer to a rectifier bridge. The thyristors can be controlled by a microcomputer that can sense various operational parameters of the precipitator and its power supply. In response to various changes in precipitator parameters, this known system can adjust the extent of drive through the power supply in anticipation of imminent sparking, thereby reducing the likelihood of sparking. As such, the microcomputer-controlled power supply can operate quickly and accurately and achieve control not readily obtainable with older voltage controllers.
It is also known to produce a high voltage by constructing a high frequency power oscillator which drives a high voltage transformer. Since the high voltage transformer operates at a relatively high frequency it can have a relatively small core, which tends to reduce fabrication costs.
In practical precipitator power supplies, the high electrostatic potential within the precipitator will occasionally cause a spark. A precursor of this spark can be a back-corona effect, wherein ions of the wrong potential tend to migrate within the field. This back-corona effect produces a negative resistance which tends to hasten a voltage breakdown or sparking condition. The instant prior to sparking exhibits a potential distribution wherein a significant potential gradient exists across any dust layer on the precipitator plates. A spark often dislodges a portion of the dust layer and creates a discontinuity in the potential gradients in the vicinity of a recent spark. It has been found that this discontinuity tends to foster further back-corona effects and sparking.
An important consideration in running a precipitator efficiently is keeping the average potential in the precipitator sufficiently high to cause a high extent of precipitation but not so high as to cause a rapid rate of sparking. It has been found that the potential across the dust layer in a precipitator does not necessarily initiate a spark instantaneously. Therefore, the possibility exists of briefly applying a high electrostatic potential during a transient period of time sufficiently short and infrequent so as to avoid excessive voltage across the dust layer. Since the dust layer is not excessively stressed by this high potential, there is a reduced likelihood of sparking.
Accordingly, there is a need for an improved power supply for an electrostatic precipitator that can operate with a high voltage that is periodically increased at a relatively low repetition rate, thereby increasing the extent of precipitation without inducing unnecessary sparking.